Interference pigment flakes and foils have been developed for a wide variety of applications such as decorative cookware, creating patterned surfaces, and security devices. Similarly, color shifting pigments have been developed for such uses as cosmetics, inks, coating materials, ornaments, ceramics, automobile paints, anti-counterfeiting hot stamps, and anti-counterfeiting inks for security documents and currency.
Color shifting pigments and foils exhibit the property of changing color upon variation of the angle of incident light, or as the viewing angle of the observer is shifted. The color shifting properties of pigments and foils can be controlled through proper design of the optical thin films or orientation of the molecular species used to form the flake or foil coating structure. Desired effects can be achieved through the variation of parameters such as thickness of the layers forming the flakes and foils and the index of refraction of each layer.
The changes in perceived color which occur for different viewing angles or angles of incident light are a result of a combination of selective absorption of the materials including the layers and wavelength dependent interference effects. The interference effects, which arise from the superposition of light waves that have undergone multiple reflections, are responsible for the shifts in color perceived with different angles. The reflection maxima changes in position and intensity, as the viewing angle changes, due to changing interference effects arising from light path length differences in the various layers of a material which are selectively enhanced at particular wavelengths.
Color shifting pigments and foils may include multilayer interference filters formed of thin film layers, each including one or more absorber layers and dielectric layers also referred to as spacer layers, and optionally one or more reflector layers, in varying layer orders.
Color shifting pigments and foils may have a symmetrical multilayer thin film structure, such as: absorber/dielectric/reflector/dielectric/absorber; or absorber/dielectric/absorber. Alternatively, foils and pigments may have an asymmetrical multilayer thin film structure, such as: absorber/dielectric/reflector.
A color shifting foil including the multilayer thin film structure may be formed by deposition of the layers onto a flexible substrate, e.g. PET web, by methods well known in the art of forming thin film coatings, such as PVD, sputtering, or the like.
The multilayer thin film structure may also be formed on a web coated with a release layer, and then removed from the web material and broken into color shifting flakes, which can be added to a polymeric medium such as various pigment vehicles for use as an ink or paint. Various thin-film flakes and methods of manufacturing such flakes are disclosed e.g. in U.S. Pat. Nos. 4,838,648, 7,258,915, 6,838,166, 6,586,098, 6,815,065, 6,376,018, 7,550,197, 4,705,356, 5,135,812, 5,571,624, all of which are incorporated herein by reference.
To enhance the counterfeit resistance and the ease of visual authentication of documents protected by optically variable security elements, it has been proposed to use paired color shifting structures which exhibit a same color at one viewing angle, and different colors—at a second viewing angle. In such a device, a uniformly colored image may explode in two or more colors when tilted. U.S. Pat. Nos. 5,766,738, 6,114,018, and 6,472,455, which are incorporated herein by reference, disclose paired structures. By way of example, two types of pigment flakes may be used for making two ink coatings matching in color at one angle and exhibiting different colors at another angle. The both types of flakes have the absorber/dielectric/reflector/dielectric/absorber design, however differ in the thickness of the dielectric layer.
The difference in color may be quantified by using color coordinates, e.g., in terms of C* (chroma), h (hue), and L* (lightness). One method of designing paired optical structures is using a quarter-wave or half-wave design, wherein thicknesses of the dielectric layers are different multiples of a quarter-wave or a half-wave thickness based on selected design wavelengths. Some known methods for designing paired interference structures concentrate on matching in hue at a particular angle. Paired pigments are considered to be matching at a certain angle even when they mismatch in chroma and/or lightness. It has been suggested to minimize variations in lightness and chroma by adding a black or neutral transparent pigment to one of the inks. However, depending on the printing method, such additions could affect the print quality. In view of the foregoing, it may be understood that there are significant problems and shortcomings associated with current solutions and technologies for providing paired interference structures that match in hue, chroma, and lightness at a particular angle of observation and mismatch at another angle.